1. Field of the Invention
This invention relates to a process for preparing an electrical connecting member to be used in connecting electrically electrical circuit parts to each other.
2. Related Background Art
In the prior art, as the method for connecting electrically electrical circuit parts to each other, there have been known the wire bonding method, TAB (Tape Automated Bonding) method, etc. However, in these methods, there have been involved the drawbacks that they cannot correspond to the increase of connecting points between the both electrical circuit parts, and are also high in cost. In order to solve such drawbacks, there has been known in the art to connect electrical circuit parts to each other by use of an electrical connecting member having a constitution equipped with a plurality of electroconductive members insulated from each other in an insulating holding member (Japanese Laid-open Patent Applications Nos. 63-222437, 63-224235, etc.).
FIGS. 1A and 1B illustrate schematically electrical connection between the electrical circuit parts by use of such electrical connecting member, and in the FIG. 31 shows the electrical connecting member, and 32, 33 show the electrical circuit parts to be connected. The electrical connecting member 31 is constituted of a plurality of electrical members 34 comprising a metal or an alloy equipped in the holding member 36 comprising an electrically insulating material wherein the electroconductive members 34 are electrically insulated from each other, and has one end 38 of the electroconductive member 34 exposed on one electrical circuit part 32 side, and the other end 39 of the electroconductive member 34 exposed on the other electrical circuit part 33 side (FIG. 1A). To the connecting portion 36 of one electrical circuit part 32 is bonded one end 38 exposed on the electrical circuit part 32 side of the electroconductive member 34 by alloy formation, and to the connecting portion 37 of the other electrical circuit part 33 is bonded the other end 39 exposed on the electrical circuit part 33 side of the electroconductive member 34 by alloy formation (FIG. 1B).
In such electrical connecting member, there are the advantages as shown below.
(1) By making the size of the electroconductive member finer, the connecting portion of the electrical circuit part can be made compact, and also for such reason, the connecting points can be increased, whereby higher density connection mutually between electrical circuit parts can be done.
(2) Even for electrical circuit parts with different thickness, the height of the electrical circuit parts can be made always constant by varying the thickness of the electrical connecting member, whereby multi-layer connection can be done easily to enable higher density practical mounting.
(3) By making the protruded height of the electroconductive member to be connected with the connecting portion of the electrical circuit part higher, even one with the connecting portion of the electrical circuit part sunk from the surface can be stably connected, whereby it is possible to connect electrical circuit parts easily with complicated shapes to each other.
(4) Since the heat generated from the electrical circuit parts is radiated through the holding member of the electrical connecting member, the electrical characteristic change is extremely small. Also, because of excellent characteristic of radiation of heat, the influence of thermal fatigue is small and reliability is also high. At this time, the holding member may be also constituted of an insulating material in which high thermal conductive metals or ceramics are mixed, and by use of insulating ceramics with high thermal conductivity, its effects becomes greater.
(5) Since the connecting length in connection mutually between the electrical circuit parts is short, impedance can be reduced and speed-up of electrical parts can be effected. By mixing a metal with low dielectric constant, etc. into the holding member, impedance can be further lowered.
As the process for preparing the electroconductive member as described above for performing electrical multi-point connection mutually between electrical circuit parts, there is one proposed in Japanese Laid-open Patent Application No. 02-49385. In the following, this preparation process is described briefly by referring to FIGS. 2A to 2F showing schematically its steps.
First, a base member 51 comprising a metal sheet such as copper foil, etc. is prepared (FIG. 2 A), and on the base member 51 is coated a photosensitive resin 52 by a spin coater, which is then pre-backed at a temperature around 100.degree. C. (FIG. 2B). Through a photomask forming a predetermined pattern (not shown), light is irradiated (exposed) on the photosensitive resin 52, followed by developing. At the irradiated portion, the photosensitive resin 52 remains, while at the unirradiated portion, the photosensitive resin 52 is removed by the development, whereby a plurality of holes 53 are formed (FIG. 2C). After curing the photosensitive resin 52 by elevating the temperature to 200.degree. to 400.degree. C., the base member 51 is dipped in an etchant to effect etching, thereby forming concavities 54 communicated to the holes 53 on the base member 51 (FIG. 2D). This etching is chemical etching by use of a general etchant such as cupric chloride, ammonium persulfate, ferric chloride, etc. Next, by application of gold plating with the base member 51 as the common electrode, gold 55 is filled into the holes 53, the concavities 54, and the gold plating is continued until bumps are formed (FIG. 2E). Here, as the means for performing gold plating, there have been known the following methods:
(1) the method by use of a mesh electrode as the counter electrode (FIG. 3A).
(2) the method by use of a foil electrode as the counter electrode (FIG. 3B).
(3) the method by use of an electrode provided with a plurality of nozzles as the counter electrode (jet stream system plating system) (FIG. 3C);
(4) the method disclosed in Japanese Patent Publication No. 57-9636.
Finally, the base member 51 is removed by etching to prepare an electrical connecting member 31 (FIG. 2F).
In the electrical connecting member 31 thus prepared, the gold 55 constitutes the electroconductive member 34, and the photosensitive resin the holding member 35. The dimensions of the respective portions in the electrical connecting member 31 are made about 10 .mu.m for the thickness of the photosensitive resin 52 (holding member 35), about 20 .mu.m for the diameter of the hole (electroconductive member 34), about 40 .mu.m for pitch, and about several .mu.m for protruded amount at both front and back of the electroconductive member 34.
However, in the preparation process of the prior art proposed in Japanese Laid-open Patent Application No. 02-49385 as described above, for mass production of electrical connecting members with uniform protruded shape and size of the bumps, the problems to be solved as described below remain and there was left room to be further improved.
Because electrical connecting members are prepared only on surface of the base member, during the steps of partial etching for formation of concavities and gold plating for filing gold, the other surface has been masked. Such masking necessitated batch treatment one sheet by one sheet, whereby there have been involved the problem that the treatment speed in etching, gold plating steps which are especially time consuming steps are very slow.
There also ensued such problems that etchant penetrated through slight gaps in such masking to each abnormally the base member, or that gold plating liquid was penetrated through the gaps to effect abnormal growth of gold (electroconductive member) by the change in current density, or that the base member cannot be removed because gold was precipitated on the other surface of the base member.
The base member has the function as the base foil for coating a photosensitive resin thereon, the function as the mold which becomes the protruded shape of the electroconductive member and the function as the common electrode for gold plating during formation of an electroconductive member, respectively, in every step, which, however, were not essentially required for the electrical connecting member finally prepared. Moreover, the thickness of the base member necessary as the mold in protruded shape of electroconductive member is about some .mu.m, which means that only a little portion of the thickness of the substrate as a whole is utilized, and most portions have been discarded without use. Therefore, there has been involved the problem that the production cost was high on account of such many superfluous steps.
Mass production of electrical connecting members with uniform protruded height and protruded amount of the respective electroconductive members depends on the skill of electroplating in the preparation steps of electrical connecting members, but there have been involved such problems as shown below.
In the methods of (1) and (2), it is difficult to control the plating amount, whereby the plating thickness and plating shape in the respective pores are ununiform, and the protruded height and the protruded amount are uniform in the respective electroconductive members. Particularly, such tendency is marked when forming fine electroconductive members.
In the method of (3), although high speed plating is possible, since the flow speed is different between the central portion and the peripheral portion of the matrix member, and also between the portion against which the jet stream from nozzles comes and the portion positioned between adjoining nozzles, the plating thickness does not become uniform. Particularly, such tendency is marked when forming fine electroconductive members.
In the method of (4), it is not necessarily possible to accomplish layering of plating liquid, and in such case, the protruded height of the respective electroconductive members may be sometimes ununiform. Also, the apperatus constitution is bulky, which poses a problem particularly in that the matrix member to be foild is of large scale. Further, when impurities are entrained in the plating liquid, it is necessary to exchange frequently the plating liquid, resulting in increased cost.
Thus, the respective electroplating methods employed in the art had the problems as described above.
On the other hand, for controlling the protruded shape of the electroconductive member, a metal sheet constituted of a laminate comprising a plurality of layers with different materials may be sometimes used. In such case, if an etchant having a constant etching speed in the respective layers is employed, there has been the problem that the protruded shape of the electroconductive member can be controlled with difficulty, unless the etching conditions in the respective layers are changed.
The photosensitive resin 52 to be coated on the base member 51 comprising a metal sheet is a liquid resing containing a solvent, and the film thickness of the photosensitive resin 52 differs before curing and after curing due to dissipation of the solvent and curing shrinkage of the resin itself. Therefore, when the photosensitive resin 52 is coated with the same thickness as the desired film thickness of the holding member 35, there has been involved the problem that no holding member 35 with the disired film thickness could be obtained.
Further, in the preparation steps of electrical connecting members when the energy density of exposure is high, in nega-type photosensitive resin, the residula resin tends to become smaller in amount, while when the exposure energy density is lower, in the nega-type photosensitive resin, the residual resin tends to become larger in amount, whereby there has been involved the problem that no hole with desired shape can be formed after developing. On the other hand, when the developing time is long, in the nega-type photosensitive resin, the residual resin tends to become smaller in amount, while when the developing time is short, the portion to be removed by developing remains, whereby there has been involved the problem that no hole with desired shape can be formed.
Further, in the preparation process of the prior art, the concavities 54 are formed by etching the surface of the base member 51 comprising a metal sheet exposed within the holes 53 of the holding member as the mask, but if the photosensitive resin 52 is thick, etchant is liable to reside in the holes 53, and hydrogen gas generated during etching is adsorved as bubbles on the base member 51 surface to cover thereover and inhibit contact with the etchant or circulation of the etchant and therefore the etching speed is slow, whereby there were problems that the shapes of the concavities 54, and hence the protruded portion shapes of the bumps were ununiform and also electrical characteristics were not stabilized, etc. Residence of the etchant within the holes 53 will also cause resinous residue to be formed at the bottom within the holes 53 of the photosensitive resin 52, and in such case, variety occurred in the protruded shapes of the respective electroconductive members, whereby there was the problem that electrical connecting members could not be prepared with good precision.
Further, if a base member 51 made of a metal having crystal grain boundary such as copper, etc. is attempted to be etched by chemical etching, etching occurs on the crystal grain boundary and/or the crystal face susceptible to ethcing, and unevenness by crystal grains remained on the surface of concavity 54 after etching. For this reason, as the result of formation of unevenness also on the surface of the bump 34b formed by plating in the concavity 54, the shapes and heights of the respective bumps 34b became ununform. Particularly, the influence by the crystal grain boundary is great when the bumps become finer and higher in density, whereby there was the problem that no uniform connection could be effected with electrical circuit parts on the bump 34b side.
Also, according to the preparation process of the prior art, during formation of the electroconductive member 34 by gold plating, initially nucleation of gold occurred in the base memberr 51, and the metal (copper) in the base member 51 is penetrated before the gold 55 covers the base member 51 exposed, whereby the gold 55 and the metal (copper) in the base member 51 were co-precipitated. The electroconductive member 34 thus formed has a co-precipitated layer remaining at the protruded portion on the base member side, and when bonded to the connecting portion of electrical circuit part, there was the problem that variety of bonding strength or defective bonding was occurred.
In the case where an amount corresponding to a plurality of sheets is prepared comprehensively on one sheet for enhancing production efficiency of the sheet-shaped electrical connecting member 31, and divided after completion, when the photosensitive resin 35 at the dividing position is attempted to be cut, a stress occurs in the vicinity of the cutting portion, whereby cracks, etc. are generated to bring about drop-off or defective connection of the eletroconductive member 34, thus involving the problem that it can be cut with difficulty.
The holding member 35 of the electrical connecting member 31 as described above generally employs a photosensitive resin such as polyimide resin, etc., and its thickness is usually about 2 .mu.m to 1 mm. Accordingly, in conveying or handling of the electrical connecting member 31, if excessive force is applied on the holding member 35, there were the problems that damages such as breaking, etc. were broughy about, that the electrocondcutive member 34 was dropped off by the stress occurring in the holding member 35.
Further, in the course of preparation as described above, in the photosensitive resin 35 coated on the base member 51, the solvent component contained in the liquid resin during precure is scattered to cause shrinkage to occur, and also internal stress occurs because of the difference in thermal expansion ratio between the resin and the base member. For this reason, when the base member 51 supporting the photosensitive resin 35 is removed, only the photosensitive resin 35 must stand the internal stress, and if further external force is applied, cracks may be formed or breaking occurs, whereby there was a fear that drop-off of electroconductive member 34 might be brought about.